Project Summary The neural circuits controlling motor behaviors vital to mammals, including walking, breathing, and balance, rely on the ability of neurons within the spinal cord to establish selective connections during development. Work over the past decade has provided a fairly comprehensive understanding of the genetic pathways that determine the identity of each major neuronal class within the neural tube. The mechanisms through which neurons acquire subtype identities necessary for the incorporation into a particular motor circuit are, however, still poorly defined. Our studies on the specification of spinal motor neurons indicate that the large family of Hox transcription factors play key roles in generating the hundreds of subtypes required for selective innervation of muscle. Hox proteins orchestrate genetic programs that control diverse aspects of motor neuron maturation, including their topographic organization, peripheral target muscle specificity, and presynaptic partners. Emerging studies from our group also indicate that Hox genes function in multiple neuronal classes to shape synaptic specificity during development, suggesting a broader role in circuit assembly. The overall goals of the proposed research are to elucidate the mechanisms of neural diversification and connectivity within the spinal cord, and to determine how Hox-dependent and -independent genetic programs establish the circuit architectures necessary for motor control. Ultimately, we hope to uncover the pathways through which genetically encoded developmental programs contribute to the emergence of specific motor behaviors. Our approach integrates selective genetic manipulations of neuronal subtypes, genome-wide interrogation of regulatory networks, modern circuit-tracing methods, comparative analyses in multiple vertebrate organisms, and rigorous analyses of behavior. Elucidating the basic mechanisms of motor circuit assembly will provide foundational insights relevant to the design of therapeutic strategies to treat degenerative diseases of spinal neurons or repair motor circuits damaged by injury.